In recent years, widely used in motor vehicle air conditioners in place of conventional serpentine condensers are condensers which comprise, as shown in FIG. 16, a pair of headers 80, 81 arranged in parallel and spaced apart from each other, parallel flat refrigerant tubes 82 made of aluminum and each joined at its opposite ends to the two headers 80, 81, corrugated aluminum fins 83 each disposed in an air flow clearance between adjacent refrigerant tubes 82 and brazed to the adjacent tubes 82, an inlet pipe 84 connected to the upper end of peripheral wall of the first 80 of the headers, an outlet pipe 85 connected to the lower end of peripheral wall of the second 81 of the headers, a first partition 86 provided inside the first header 80 and positioned above the midportion thereof, and a second partition 87 provided inside the second header 81 and positioned below the midportion thereof, the number of refrigerant tubes 82 between the inlet pipe 84 and the first partition 86, the number of refrigerant tubes 82 between the first partition 86 and the second partition 87 and the number of refrigerant tubes 82 between the second partition 87 and the outlet pipe 85 decreasing from above downward to provide groups of channels. A refrigerant flowing into the inlet pipe 84 in a vapor phase flows zigzag through the units of channel groups in the condenser before flowing out via the outlet pipe 85 in a liquid phase. The condensers of the construction described are called multiflow condensers, and realize high efficiencies, lower pressure losses and supercompactness.
It is required that the refrigerant tube 82 of the condenser described be excellent in heat exchange efficiency and have pressure resistance against the high-pressure gaseous refrigerant to be introduced thereinto. Moreover, the tube needs to be small in wall thickness and low in height so as to make the condenser compact.
A flat tube outstanding in heat exchange efficiency and adapted for use as such a refrigerant tube 82 is already known which comprises an upper and a lower wall, a right and a left side wall interconnecting the upper and lower walls at the respective right and left side edges thereof, and a plurality of reinforcing walls interconnecting the upper and lower walls, extending longitudinally of the tube and spaced from one another as positioned between the right and left side walls, the tube having parallel fluid channels formed inside thereof, each of the reinforcing walls being formed from a downward ridge projecting downward from the upper wall integrally therewith and an upward ridge projecting upward from the lower wall integrally therewith by brazing the two ridges to each other as butted against each other (see the publication of Japanese Patent No. 2915660, FIG. 4).
Such a flat tube is fabricated from a single sheet of metal product of miscellaneous cross section having two flat wall forming portions connected to each other by a connecting portion, a side wall ridge projecting from each flat wall forming portion integrally therewith at a side edge thereof opposite to the connecting portion and reinforcing wall ridges projecting inward from each flat wall forming portion integrally therewith, by bending the product to the shape of a hairpin at the connecting portion to butt the side wall ridges, as well as each pair of corresponding reinforcing wall ridges, to each other, and brazing the butted ridges in corresponding pairs.
Another flat tube outstanding in heat exchange efficiency and adapted for use as such a refrigerant tube 82 is also known as disclosed in the above publication. This flat tube comprises an upper and a lower wall, a right and a left side wall interconnecting the upper and lower walls at the respective right and left side edges thereof, and a plurality of reinforcing walls interconnecting the upper and lower walls, extending longitudinally of the tube and spaced from one another as positioned between the right and left side walls, the tube having parallel fluid channels formed inside thereof, each of the reinforcing walls being formed from a reinforcing wall ridge projecting inward from at least one of the upper and lower walls integrally therewith, by brazing the ridge to the flat inner surface of the other wall.
Such a flat tube is fabricated from a single sheet of metal product of miscellaneous cross section having two flat wall forming portions connected to each other by a connecting portion, a sidewall ridge projecting from each flat wall forming portion integrally therewith at a side edge thereof opposite to the connecting portion and reinforcing wall ridges projecting from at least one of the flat wall forming portions integrally therewith in the same direction as the side wall ridge thereon, by bending the product to the shape of a hairpin at the connecting portion to butt the side wall ridges against each other and to bring the outer ends of the reinforcing wall ridges on the above-mentioned one flat wall forming portion into contact with the other flat wall forming portion, and brazing the side wall ridges to each other and the outer ends of the reinforcing wall ridges to the other flat wall forming portion.
As disclosed in the above publication, the product of miscellaneous cross section described is manufactured from a brazing sheet having a brazing material layer on opposite sides thereof by passing the sheet through a rolling apparatus comprising a first work roll provided with ridge forming annular grooves over the entire circumference thereof for forming the side wall ridges and the reinforcing wall ridges, and a second work roll having a smooth cylindrical peripheral surface.
In the case where rolled sheets are manufactured by the rolling apparatus having two work rolls, both the work rolls are generally made from a high-speed tool steel and have respective peripheral surfaces of the same diameter. When the above-mentioned product of miscellaneous cross section is to be manufactured by passing a brazing sheet through a rolling apparatus comprising a first work roll provided with ridge forming annular grooves over the entire circumference thereof for forming the side wall ridges and the reinforcing wall ridges, and a second work roll having a smooth cylindrical peripheral surface, the portions of peripheral surface of the first work roll where no ridge forming annular grooves are formed are conventionally given a diameter equal to the diameter of the peripheral surface of the second work roll.
However, this rolling apparatus has the problem that the bottom portions of the ridge forming annular grooves in the first work roll wear away at a rate higher than is estimated.
In order to inhibit the wear of work rolls having a smooth cylindrical peripheral surface, it has been proposed to form in the peripheral surface of the roll minute surface irregularities providing a reservoir for the rolling oil (see the publication of JP-A No. 1998-166010).
Means nevertheless has yet to be found for inhibiting the wear on the bottom portions of the ridge forming grooves, for use in the work roll having these grooves for making products of miscellaneous cross section such as the one described above.
An object of the present invention is to overcome the above problem and to provide a rolling apparatus comprising a work roll which has ridge forming annular grooves in its peripheral surface and which is inhibited from wearing away at the bottom portions of the grooves, and a method of manufacturing a product of miscellaneous cross section with use of the apparatus.